1. Field of the Invention
The present invention relates to an electric power supply device for a monitor, particularly to a power supply device which can reduce power consumption of a monitor when a synchronizing signal is not inputted.
2. Description of the Prior Art
A power supply device of a monitor is a device which converts an alternating current AC inputted from outside into a direct current DC which is then loaded to a microprocessor, a video processor, a heater, etc. A conventional power supply device of a monitior is shown in FIG. 1. In FIG. 1, reference numeral 1 refers to an electric power source at which an alternating current inputted from outside is subject to full-wave rectification and smoothing, and is supplied to a primary winding L11 of a transformer T; reference numeral 2 refers to a first power supply section at which an excitation voltage of a first secondary winding L21 of the transformer T is subject to half-wave rectification and smoothing, and is supplied to a first load LD1; reference numeral 3 refers to a second power supply section at which an excitation voltage of a second secondary winding L22 of the transformer T is subject to half-wave rectification and smoothing, and is supplied to a second load LD2; reference numeral 4 refers to a power economy section at which a mode switching signal CL is generated according to the presence of a synchronizing signal inputted from outside, and an output voltage Vo1 of a first power supply section 2 is supplied to a second power supply section 3 according to a mode switching signal CL; reference numeral 5 refers to a control section at which a control signal, which is initiated by an output voltage of the electric power source 1 for allowing flow of a current through the primary winding L11 of the transformer T, is generated, and the control signal is varied according to a current supplied from outside; and finally, reference numeral 6 refers to a switching section, which is switched according to a control signal of the control section 5 to control the current flowing through the primary winding L11 of the transformer T, and which includes a transistor for switching.
In a power supply device of a monitor as described above, to an output terminal of a bridge diode BD of the electric power source 1 at which an inputted alternating current is rectified, an input terminal of a capacitor Cin at which an output voltage of the bridge diode BD is smoothed and supplied to the primary winding L11 of the transformer T is connected. Another terminal of the capacitor cin is connected to earth.
To one end of the first secondary winding L21 of the transformer T, anode of a diode Do1 of the first power supply section 2 for rectifying an excitation voltage of the first secondary winding L21 of the transformer T is connected; to cathode of the diode Do1, one end of the capacitor Co1 for smoothing the output voltage of the diode Do1 is connected; and another end of the capacitor Co1 as well as another end of the first secondary winding L21 of the transformer T are connected to ground.
Further, one end of the secondary winding L22 of the transformer T is connected to anode of the diode Do2 of the second power supply section 3 at which an excitation voltage of the second secondary winding L22 of the transformer T is subject to half-wave rectification; to cathode of the diode Do2, one end of the capacitor Co2 at which the output voltage of the diode Do2 is smoothed and supplied to the second load LD2; and another end of the capacitor Co2 is connected to one end of a regulator at which the output voltage of the capacitor Co2 is regulated to be a constant voltage. Another end of the second secondary winding L22 of the transformer T is connected to ground.
In the meantime, an output terminal of the second load LD2 is connected to a base terminal of a transistor Q41 of the power economy section 4 to switch a mode swithing signal CL ouputted from the second load LD2; to a collector of the transistor Q41, an output terminal of a resistance R41 for controlling an output voltage of the transistor Q41 is connected; and an emitter of the transistor Q41 is connected to ground.
An input terminal of a resistance R41 is connected to a base terminal of the transistor Q42; an emitter of the transistor Q42 is connected to an output terminal of a resistance R42; and a collector of the transistor Q42 is connected to an input terminal of a regulator REG of the second power supply section 3. An input terminal of the resistance R42 is connected to an output terminal of the first power supply section 2.
In a power supply device of a monitor as described above, an alternating current AC inputted from outside is supplied to a bridge diode BD of the electric power source 1 and is subject to full-wave rectification. An output voltage of the bridge diode BD is supplied to the capacitor Cin and smoothed. An output voltage of the capacitor Cin is supplied to the primary winding L11 of the transformer T.
In the meantime, an output voltage of the bridge diode BD is suppled to a control section 5 at which a control signal is generated and supplied to a switching section 6. The switching section 6 is switched to a turn-on state. Then a current flows through the primary winding L11 of the transformer T, and the first secondary winding L21 and the second secondary winding L22 of the transformer T are excited. At this time, the magnitudes of the excitation voltages of the first secondary winding L21 and the second secondary winding L22 of the transformer T are determined according to the turn ratio of the first secondary winding L21 and the second secondary winding L22.
Further, an excitation voltage of the first secondary winding L21 of the transformer T is supplied to a diode Do1 of the first power supply section 2, after which the excitation voltage of the first secondary winding L21 of the transformer T is rectified at the diode Do1. An output voltage of the diode Do1 is supplied to a capacitor Co1 at which an output voltage of the diode Do1 is smoothed, and an output voltage of the capacitor Co1 is supplied to the first load LD1. The first load LD1 is then operated.
In the meantime, an excitation voltage of the second secondary winding L22 is supplied to a diode Do2 of the second power supply section 3; an excitation voltage of the second secondary windig L22 is subject to half-wave rectification at the diode Do2; an output voltage of the diode Do2 is supplied to a capacitor Co2 at which an output voltage of the diode Do2 is smoothed. An output voltage of the capacitor Co2 is supplied to a regulator REG at which the output voltage of the capacitor Co2 is converted to be a constant voltage. An output voltage of the regulator REG is supplied to a second load LD2. The second load LD2 is then operated.
Under this circumstance, once a synchronizing signal SYN is inputted to the second load LD2 from outside, a low-potential mode switching signal CL is generated at the second load LD2 and is supplied to a transistor Q41 of a power economy section 4. The transistor Q41 is then switched to a turn-off state. Accordingly, another transistor Q42 is switched to a turn-off state, after which output voltages of the first power supply section 2 and the second power supply section 3 are supplied to the first load LD1 and the second load LD2, respectively.
However, if a synchronizing singnal SYN is not inputted from outside, a control signal of the control section 5 is varied since a voltage is supplied from outside to the control section 5 in order to stop driving of the loads except for a second load LD2 of a monitor. Therefore, the quantity of the current flowing through the primary winding L11 of the transformer T is varied, and accordingly, excitation voltages at the first secondary winding L21 and the second secondary windig L22 are reduced to about one tenth of their normal values, and the first load is not operated.
A high-potential mode switching signal CL is generated at the second load LD2, after which a transistor Q41 of a power economy section 4 is switched to a turn-on state as well as another transistor Q42. In other words, the voltage supplied from the first power supply section 2 to the first load LD1 is supplied to a regulator REG of the second power supply section 3 through the transistor Q42 of the power economy section 4, and an output voltages of the first power supply section 2 is converted to be a contant voltage at the regulator REG. The output voltage of the regulator REG is supplied to the second load LD2 of which operation is continued. That is, the operation of the second load LD2 is continued without input of a synchronizing signal SYN.
In a general power supply device of a monitor, in order to drive the second load LD2, a bridge diode BD, a capacitor Cin, and a transformer T have to maintain their operations even when no synchronizing signal SYN is inputted from outside, which leads to a problem that the power consumption of the transformer T is greater than that of the second load LD2.